The major objective of this research proposal is to elucidate the mechanism of control of serum albumin biosynthesis and secretion, a useful model for understanding the regulation of protein biosynthesis associated with secretion. The function in this process of a novel rat liver precursor to rat serum albumin, rat proalbumin, will be studied. This will include a characterization of the system involved in the in vitro biosynthesis of proalbumin, and a search for possible precursors of this protein. The possible function of the precursor peptide moiety in transport of the albumin polypeptide chain through the endoplasmic reticulum to the locus of secretion will be considered by a study of the relationship of proalbumin to other proteins within this system. The small peptide produced in the course of conversion to serum albumin by cleavage of proalbumin hexapeptide will be identified and characterized. Especial interest here will be in the possible involvement of the conversion peptide in the control of proalbumin biosynthesis.